If you have a period, or know anyone who does, you will have likely heard about “cycle syncing”.

There are many myths surrounding menstruation that don’t have that much evidence, and cycle syncing is probably the most pervasive one. 

It’s commonly believed that women who live or spend a lot of time together eventually “sync” cycles, meaning that they start their period at the same time every month. But does science back this up? 

What is period syncing? 

Also known as “menstrual synchrony” and “the McClintock effect”, the belief that women’s periods can line up with one another is based on the theory that when two or more people who menstruate spend a lot of time together, their pheromones influence each other so that eventually their cycles sync up. 

Most of us assume that period syncing is a real thing, and to be honest there’s no shame in that—stigma surrounding menstruation and the lack of research on gynaecological health means there is a lot of misinformation floating about. But how did cycle syncing even become a thing? 

The theory dates back to 1971, when a Harvard researcher called Martha McClintock published a study titled Menstrual Synchrony and Suppression in the scientific journal Nature. Her research studied 135 women living in student accommodation and found that, over the course of the academic year, roommates and close friends were more likely to “sync” cycles compared to random pairings of dorm students.  

McClintock hypothesised that roommates spent more time together, therefore their pheromones communicated with each other due to the close proximity. She also based her findings on what she called the “alpha uterus” theory. Yes, you read that correctly. According to her study, an alpha uterus has a “hormonal pull” that dictates over other cycles, triggering cycle syncing. 

Do women’s periods really sync up? 

Many researchers have since pointed out faults in McClintock’s original study. For one, 135 is a small sample size, and the research was only based on the analysis of 8 cycles per woman.  

Secondly, the results only showed a decrease in the difference between period onset dates, so the definition of “synchronisation” is a little loose. Critics also pointed out that McClintock's 1971 findings didn't account for pure coincidence as to why women's cycles were in sync. Furthermore, McClintock’s study wasn’t a randomised clinical trial, but rather a one-off observational study, so it should be taken with a pinch of salt. 

Since 1971 scientists have conducted many studies on room mates, lesbian couples, best mates and even animals to see if period syncing could actually be a thing, but the results have always been mixed at best. 

A 2006 study and review analysed data from 186 women living in a dorm and concluded that “women do not sync their menstrual cycles.” Rather, the study said that any period syncing that happened was nothing more than mathematical coincidence.

When you think about it, it’s incredibly common for cycle lengths to vary, and period length is different from person to person. Although we’ve been led to believe that a “normal” cycle is 28 days long (another debunked trope), the average cycle length can vary between 24 and 38 days. And even then, it’s subject to change. 

Nutrition, stress, contraception, travel, pregnancy and general health can all influence the length or your cycle, so it’s very likely that sometimes you’ll start your period at the same time as your roommate, partner, sibling or friend. Basically, period overlapping is best explained by the law of probability. 

Some studies have even concluded that due to the variability in cycle length among women, period synchronisation is a “mathematical impossibility”.  

The most recent (and largest to date) study was conducted by Oxford University and the period tracking app, Clue, and largely debunked McClintock’s findings. They collected data from over 1,500 people and found that women can’t really disrupt each other’s cycles by being in close proximity to one another. 

Of the 1,500, Clue narrowed it down to 360 pairs and found that “273 pairs (76 percent of the sample) actually had a larger difference in cycle start dates at the end of the study than at the beginning of the study.” Cycle start dates got closer and closer for only 79 of the pairs. 

To this day we also don’t know whether pheromones—the main theory on which period syncing hinges on—truly affect another person’s menstrual cycle. Pheromones are chemical signals that trigger a social response in members of the same species. They’re a non-verbal way of communicating with other humans and can influence attraction, fertility, sexual arousal and more. But we don’t really know whether pheromones have the power to signal when another woman's period should start. 

From a scientific standpoint, period syncing has been sufficiently debunked, but anecdotally, many people still believe in the phenomenon because it can promote bonding and a sense of sisterhood. There’s nothing like the shared experience of menstruation to make you feel close to someone else, so even though the jury is out on the validity of period syncing, we’re all in this together at the end of the day.